Parallel effects of land‐use history on species diversity and genetic diversity of forest herbs

The two most fundamental levels of biodiversity, species diversity and genetic diversity, are seldom studied simultaneously despite a strikingly similar set of processes that underlie patterns at the two levels. Agricultural land use drastically reduces populations of forest herbs in the north‐temperate zone, so that bottlenecks or founder events in forests on abandoned agricultural land (i.e., secondary forests) may have a long‐term impact on both species diversity and genetic diversity. Using forest‐herb community surveys and molecular‐genetic analysis of populations of Trillium grandiflorum, a representative species of forest herb, I investigated the influence of land‐use history, landscape context, and environmental conditions on diversity and divergence at the population and community levels. Secondary forests (70–100 years old) had reduced diversity of both genes and species relative to primary forests (i.e., stands never cleared for agriculture). The community in secondary forests had an overrepresentation of the most common species in the landscape, though divergence in species' relative abundances within stands suggested an influence of community drift via local bottlenecks. Secondary‐forest populations of T. grandiflorum were more genetically divergent than those in primary forests, again indicating drift in small populations. Land‐use history and the size of populations and communities drove correlations between species diversity and genetic diversity (and community divergence × genetic divergence), though the strength of correlations was relatively weak. These results extend the generality of positive species–genetic diversity correlations previously observed for islands, and they demonstrate a long‐term legacy of land‐use history at multiple levels of biodiversity.